Method of and apparatus for stretch forming sheet metal



March 20, 1956 HQLOPCHAK 2,738,824

METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL Filed June 11,1952 3 Sheets-Sheet l INVEN TOR.

GEORGE HOLOPCHAK Agent March 1956 G. HOLOPCHAK 2,738,824

METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL Filed June 11,1952 3 Sheets-Sheet 2 IN VENTOR. GEORGE HOLOPGHAK BY March 20, 1956HOLOPCHAK METHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL 3Sheets-Sheet 3 Filed June 11, 1952 INVENTOR. GEORGE HOLOPCHAK gentMETHOD OF AND APPARATUS FOR STRETCH FORMING SHEET METAL GeorgeHolopchak, Los Angeles, Calif., assignor to Lockheed AircraftCorporation, Burbank, Calif.

Application June 11, 1952, Serial No. 292,917

8 Claims. (Cl. 15332) This invention relates to the shaping or formingof sheet metal and relates more particularly to contour clamps foravoiding the formation of unwanted wrinkles in sheet metal when stretchforming the same.

Stretch presses have gone into extensive use, particu larly in aircraftproduction, to shape or form sheet metal parts to the desired contours.It is a common occurrence for severe wrinkles to develop in the sheetparts during the stretch forming operations, particularly where theparts are formed with a large or substantial radius of curvature in thelongitudinal direction. The tendency for the wrinkles to develop isaccentuated in the case of 75S. aluminum alloys and the other sheetmetals that are inherently difiicult to form. The wrinkles must, ofcourse, be removed by subsequent hammering operations, or the like,which materially add to the cost of production and where the Wrinklesare excessively large or severe the parts may have to be discarded.

It is a general object of this invention to provide simple, practical,and inexpensive floating contour clamps for greatly reducing oreliminating the formation of unwanted wrinkles in sheet metal partsduring the stretch forming of the same. The clamps of the invention thusavoid the necessity of subsequent hammering operations to remove suchwrinkles and avoid the possibility of irreparably damaging the sheets.

Another object of theinvention is to provide clamps or devices that areapplied to the sheet metal part immediately prior to the stretching ofthe same and that serve to pre-form the sheet by imparting a transversecurvature to the same so that upon stretch forming the part or sheeteach longitudinal strip or region of the sameis given substantially thesame percentage or amount of elongation as every other region, thusavoiding or reduc ing to a minimum the wrinkles that would otherwisedevelop during the stretch forming of the part.

A further object of the invention is to provide clamps or devices ofthis kind that are simple and inexpensive to manufacture and easy andconvenient to use. Each clamp is a simple device including two hingedlyconnected sections of wood, or other suitable material, readily hung orengaged on the sheet and then clamped together beyond and adjacent theends of the die to preshape the sheet with the desired or necessarytransverse curvature. The arrangement of the clamps in position and theclosing andtightening of the same are operations that may be simply andeasily performed.

Other objectives and features of the invention will become apparent fromthe following detailed description of typical preferred embodiments,throughout which reference will be made to the accompanying drawings,where- 1n:

Figure l is a front perspective view of a stretch press showing a sheetmetal part engaged in the jaws thereof, and illustrating one form ofcontour clamps of the invention associated with the sheet, one clampbeing closed and the other clamp being operated to the closed position;

ice

Figure 2 is a front elevation of the stretch press showing another formof clamps of the invention arranged on the sheet or part;

Figure 3 is an enlarged side or end view of one of the clamps appearingin Figure 2;

Figure 4 is an enlarged side or end view of one of the clamps shown inFigure 1 illustrating the clamp in the closed position on a work part orsheet;

Figure 5 is an enlarged fragmentary front elevation illustrating one ofthe clamps, such as shown in Figure 1, connected or suspended from oneof the jaws of the stretch press; and

Figure 6 is a fragmentary enlarged perspective view illustrating themanner of mounting a clamp of the invention on an end of a die.

The devices of the invention may, of course, be constructed for use informing or shaping sheet metal parts of various kinds and may beemployed with stretch presses of different types and sizes. Theconfiguration and sizes of the devices or clamps will depend too uponthe size and character of the workpieces to be shaped or formed. In thedrawings I have shown typical devices or clamps of the invention engagedon sheet metal workpieces which are being shaped or formed in a stretchpress of a conventional type. It is to be understood that the inventionis not to be considered as restricted for use on work parts of this kindor for use with any particular type of stretch press.

The stretch press shown in the drawings includes a frame or body 10 andthe die 11 is mounted or secured on the front of this body. The die 11presents an active forward face curved both longitudinally andtransversely. The machine or press further includes two jaws 12 forgripping the ends of the sheet or work W. The jaws 12 are carried andoperated by cylinder and piston devices 13 operable to move the jawsoutwardly and thus exert a pull on the work sheet W to stretch the same.The assemblies of the jaws 12 and cylinder and piston devices 13 arecarried by triangular frames 14 hinged or pivoted to the body 10 of thepress at 15 to swing about vertical axes. Cylinder and piston devices 16are connected with these frames 14 and are operable to swing or pivotthe same and thus wrap the workpiece W onto the die 11 before theworkpiece is stretched. The stretch forming machine or press of courseincludes other parts and instrumentalities common to devices of this 7kind.

Figures 2 and 3 of the drawings illustrate a simple embodiment of theinvention suitable for use on relatively narrow sheets or workpieces W.As shown in Figure 2, two devices or clamps C of the invention areengaged on the work W, one clamp C being positioned on the work beyondbut adjacent each end of the die 11. The clamps C may be identical andeach includes two jaws or sections 29 and 21, see Figure 3. The sections20 and 21 are formed of wood, such as hard wood plywood, or any otherappropriate material. In making the clamp sections 20 and 21, a piece ofplywood, or other selected material, of rectangular, or other shape, iscut with a saw, or the like, into the two sections 20 and 21 in such amanner that the cut thus made is approximately the same as thetransverse curvature of the end portions of the die 11. Thus, as shownin Figure 3, the saw cut provides the clamp section 20 with a convexinner surface or edge 22 for engaging the inner surface of the work W,and provides the clamp section 21 with a concave inner edge 23 forengaging the outer surface of the work W and the curvature of theseedges 22 and 23 may approximate the transverse curvature of the die 11adjacent its ends. However, the curvature of the surfaces 22 and 23 neednot beexactly or even substantially the same as the curvature of anygiven die as I have found that clamps having surfaces 22 and 23 withradii of curvature somewhat greater or somewhat smaller than those ofgiven die ends operate entirely satisfactory with such dies andtherefore a .given pair of clamps C may be used successfully withseveral different dies for forming sheet metal parts having transversecurvatures of different radii.

The clamp sections and 21 are suitably hinged together at one end. Forexample, as shown in Figure 3, a hinge 24 connects the lower ends of hesections 2 and 21 at or adjacent their active edges 22 and 23. Thehinging together of the sections 20 and 21 facilitates the readyarrangement or positioning of the clamp on the work W. Clamp means isprovided at the opposite end of the clamp C for drawing the sections 2%and 21 to gether to give the work W its pro-formed transverse curvature.This means may comprise a yoke-like clamp body 25 that is slipped overthe upper portions of the sections 20 and 21 after the same have beenswung toward one another and a clamp screw 26 threaded through a nut 27on the body 25 to engage against the edge of the seeti on 21.

in employing the contour clamps C of Figures 2 and 3, the work W isfirst arranged in the stretch press with its opposite ends gripped inthe jaws 12. The clamps C are then arranged. in place on the sheet orwork W adjacent the ends of the die 11. This is done by hinging orswinging the sections 20 and 21 together or toward one another so thatthe edge surfaces 22 and 2-3 bend the work W to have a transversecurvature and the clamps 2.5 are arranged in place and operated. Upontightening down the clamp screws 26 the clamp sections 2t and 21 bendthe sheet or work W to have a transverse curvature corresponding to thecurvature of the edges 22 and 23 and therefore substantiallycorresponding to the transverse curvature of the die ends. As the clampsC of the invention are arranged on the work W at both ends of the die11, the entire portion of the workpiece extending across the face of thedie is given the initial transverse curvature by the clamps C, at leastimmediately supplying tension to the work. After the clamps C have beenpositioned and actuated, as just described, the cylinder and pistondevices 16 are operated to wrap the sheet or work onto the die 11 andthe cylinder and piston devices 13 are then actuated to stretch the workand set the work to final contour by stretching or tensioning it beyondthe yield point, these operations being typical or conventional inoperating the type of press. During the stretch forming of the workpieceW the clamps C preserve the transverse curvature at or beyond the endsof the die 11 so that the longitudinal fibres or regions of the work Ware substantially uniformly elongated or stretched. This uniform stretchforming of the part W avoids or reduces the development of unwantedwrinkles that would otherwise be formed in the work, especially wherethe die 11 has both the transverse curvature and a substantiallongitudinal curvature, as shown.

Figures 1, 4 and 5 illustrate a somewhat different type of clamp of theinvention suitable for use on wider sheets or workpieces W. Each ofthese clamps C2 includes two jaws or sections 30 and 31 which may beconstructed in the same manner as the above described clamp sections 20and 21 and which may be substantially identical therewith. Thesesections 30 and 31 have the curved edges 32 and 33 corresponding withthe edges 22 and 23, above described, and the sections are hingedtogether at their lower edges by a hinge 34. In this form of theinvention the clamps C2 are carried by the jaws 12 of the stretch press.I have shown a channel projecting inwardly from the top of each jaw 12and a plate or bracket 41 is attached to the upper end of the section 30or" each clamp C2. Plates 42 are shiftable in the channels 40 and carryhooks or hangers 43. Coil springs 44 are connected between the hangers43 and the'brackets 41. With this arrangement the clamps C2 are carriedby the jaws 12 for easy handling and yet are free to move with the workW as the same is wrapped about the ends of the die 11 and is elongatedduring the stretch forming operations. Thus the plates 42 are shiftablein the channels 40 to allow the clamps C2 to be secured on the work W atthe desired locations and the springs 44 are adapted to yield to allowfree movement of the clamps C2 with the work W as it is wrapped andstretch formed.

The clamps C2 of Figures 1, 4 and 5, have a drum and cable means fortightening or clamping the sections 30 and 31 onto the work 'W to givethe work the desired initial transverse curvature. Each of these meansincludes a winding drum 45 rotatably mounted on a side of a clampsection 31 adjacent its upper end. Cables 46 are wound on the drums 45and have hooks 47 on their ends engageable in eyes 48 secured on thecompanion clamp sections 39. After the sections 30 and 31 have beenpartially closed against the sheet or work W, the hooks 47 are engagedin the eyes 48 and the drums 45 are rotated by suitable cranks 49 totighten the clamp sections onto the work and thus bend or shape the sameto the desired initial transverse curvature. Ratchet means 50 areassociated with the drums 45 to retain the clamp sections 30 and 31 inthe actuated positions. Upon manually releasing the ratchet means 50after the stretch forming operations have been completed, the hooks 47are released from the eyes 48 and the sections 30 and 31 are swung opento permit removal of the work W.

Figure 6 illustrates still another arrangement or embodiment of theinvention. In this case the clamps C2, which may be the same as shown inFigures 1, 4 and 5, are mounted on the ends of the die 11. Twovertically spaced horizontal arms 60, are secured to each end of the die11 by vertical hinge pins 61 removably engaged through lugs 62 on thedie and through connecting tubes 63, secured to the ends of the arms 60.This mounting or attachment of the arms permits the arms to swing aboutvertical axes. Brackets 64 are attached to the sections 3t! of theclamps C2 and have sleeves 65 on their outer ends which are slidablyengaged on the pivoted arms 60. The pivotal mounting of the arms 60 onthe pins 61 allows the clamps C2 to move or swing with the end portionsof the work W when the same is wrapped on the die and the sleeves 65 arefree to move on the arms 60 so that the clamps C2 may move with the workW as it is stretched during the stretch forming operations.

From the foregoing detailed description it will be seen that I haveprovided simple, effective clamps readily applied to the sheet or work Wadjacent the ends of the die 11. The contour clamps are operable to bendthe work transversely into substantial conformance with the end portionsof the die 11 and to retain the end parts of the work in the bent orcontoured condition during the stretch forming operations. Thispre-bending or initial contouring of the work equalizes the strain inthe material or Work so that each longitudinal strip or region of thesheet is given substantially the same percentage of elongation as theadjacent strips or regions. This strain equalizing effect has been foundto avoid or eliminate the wrinkles that would otherwise develop,particularly where the die 11 has a substantial longitudinal curvatureas well as a transverse curvature. It will be observed that in each formor embodiment of the invention the contour clamps are free floating,that is they are carried by or associated with the work W itself so asto move therewith as the work is stretch formed. The contour clamps aresimple and inexpensive to manufacture and easy and convenient to use.The employment of the contour clamps does not complicate or interferewith the normal stretch forming operations.

Having described only typical forms of the invention I do not wish to belimited to the specific details herein set forth, but wish to reserve tomyself any variations or modifications that may appear to those skilledin the art and fall within the scope of the following claims.

I claim:

1. In combination, a die having a die having an end portion with atransverse curvature and against which a sheet part is to be stretchformed, gripping means for gripping the part in an area spaced beyondsaid end of the die, two clamp sections in the region between said endof the die and the gripping means, one having a convex surface forengaging one side of the part adjacent said end portion of the die, theother having a concave.

surface for engaging the opposite side of the part adjacent said endportion of the die, the curvature of said surfaces conforming generallywith the transverse curvature of said end portion of the die, and meansfor actuating the two clamp sections toward one another to clamp saidsurfaces of the sections against the sides of the part adjacent butoutwardly of said end of the die to give the part an initial transversecurvature corresponding generally with said curvature of the die.

2. In combination; a die having an end portion with a transversecurvature and against which a sheet part is to be stretch formed, jawmeans for gripping the part in a region spaced beyond said end of thedie; two clamp sections, one jaw means for gripping the part in a regionspaced beyond said end of the die; two clamp sections,

one having a convex surface for engaging one side of the part in aregion between the jaw means and said end of the die, the other having aconcave surface for engaging the opposite side of the part in saidregion between the jaw means and said end of the die, the curvature ofsaid surfaces conforming generally with the transverse curvature of saidend portion of the die, means hingedly connecting together correspondingends of the, sections, and means for actuating the sections against thepart to clamp said surfaces of the sections against the sides of thepart adjacent but outwardly of said end of the die to give the part aninitial transverse curvature.

3. In combination; a die having an end portion with a transversecurvature and against which a sheet part is to be stretch formed, jawmeans for gripping the part in a region spaced beyond said end of thedie; two

clamp sections in the region between said end of the:

die and the jaw means, one having a convex surface for engaging one sideof the part in the region between the jaw means and said end of the die,the other having a concave surface for engaging the opposite side of thepart in said region, the curvature of said surfaces conforming generallywith the transverse curvature of said end portion of the die, meansmovably supporting the sections so as to be movable with the part, andmeans for clamping the sections onto the part so that said surfaces givethe part an initial transverse curvature.

4. In combination; a die having an end portion with a transversecurvature and against which a sheet part is to be stretch formed, jawmeans for gripping the part some distance beyond said end of the die,two clamp sections in the region between the jaw means and said end ofthe die, one having a convex surface for engaging one side of the part,the other having a concave surface for engaging the opposite side of thepart, the curvature of said surfaces conforming generally with thetransverse curvature of said end portion of the die, means on the diemovably supporting the sections adjacent said end thereof so as to bemovable with the part as it is stretch formed, and means for clampingthe sections onto the part to have said surfaces give the part aninitial transverse curvature. I

5. In combination; a die having an end portion with a transversecurvature and against which a sheet part is to be stretch formed, jawmeans for gripping the part some distance beyond said end of the die,two clamp sections, one having a convex surface for engaging one side ofthe part, the other having a concave surface for engaging the oppositeside of the part, the curvature of said surfaces conforming generallywith the transverse curvature of said end portion of the die, meansmovably supporting the sections adjacent said end of the die so as tomove with the part as the same is stretch formed, means hinging togethercorresponding ends of the sections, and winding drum and cable meansassociated with the other ends of the sections for clamping the sectionson the part to have said surfaces give the part an initial transversecurvature.

.6. In combination; a die having an end portion with a transversecurvature and against which a sheet part is to be stretch formed, jawmeans for gripping the part some distance beyond said end of the die,two clamp sections arranged in the region between the jaw means and saidend of the die, one having a convex surface for engaging one side of thepart, the other having a concave surface for engaging the opposite sideof the part, the curvature of said surfaces conforming generally withthe transverse curvature of said end portion of the die, means hingedlyconnecting corresponding ends of the sections, and screw means at theother ends of the sections for clamping them onto the part so that saidsurfaces give the part an initial transverse curvature.

7. In a stretch press having a jaw for gripping an end portion of asheet metal part the combination of; a die having a transversely curvedend portion and against which the part is to be formed, two clampsections, hinge means connecting corresponding ends of the sections,means for movably supporting the sections on the jaw to be adjacent saidend of the die, one section having a convex surface for engaging oneside of the part, the other having a concave surface for engaging theother side of the part, the curvature of said surfaces substantiallyfollowing the curvature of said end portion of the die, and means forclamping the sections onto the part so that said surfaces give the partan initial transverse curvature.

8. The method of stretch forming a sheet metal part which includesengaging a side of the part against the curved face of a die, grippingthe ends of the part with jaws, giving the part an initial transversecurvature corresponding generally with the transverse curvature of theend portions of the die by gripping and transversely bending the part inregions between the ends of the die and the jaws, and then moving thejaws to subject the part to elongating forces, while said regions of thepart remain transversely bent, to stretch form the part to a contourdetermined by the shape of the die.

References Cited in the file of this patent UNITED STATES PATENTS 20,128Pitcher Apr. 27, 1858 28,901 Pierson June 26, 1860 174,398 Wright Mar.7, 1876 417,603 Sullivan Dec. 17, 1889 569,927 Laney Oct. 20, 18962,378,413 Lermont et a1. June 19, 1945 2,446,089 Hochreiter July 27,1948 2,609,860 Kindelberger Sept. 9, 1952 FOREIGN PATENTS 494,702Germany Mar. 27, 1930

1. IN COMBINATION, A DIE HAVING A DIE HAVING AN END PORTION WITH ATRANSVERSE CURVATURE AND AGAINST WHICH A SHEET PART IS TO BE STRETCHFORMED, GRIPPING MEANS FOR GRIPPING THE PART IN AN AREA SPACED BEYONDSAID END OF THE DIE, TWO CLAMP SECTIONS IN THE REGION BETWEEN SAID ENDOF THE DIE AND THE GRIPPING MEANS, ONE HAVING A CONVEX SURFACE FORENGAGING ONE SIDE OF THE PART ADJACENT SAID END PORTION OF THE DIE, THEOTHER HAVING A CONCAVE SURFACE FOR ENGAGING THE OPPOSITE SIDE OF THEPART ADJACENT SAID END PORTION OF THE DIE, THE CURVATURE OF SAIDSURFACES CONFORMING GENERALLY WITH THE TRANSVERSE CURVATURE OF SAID ENDPORTION OF THE DIE, AND MEANS FOR ACTUATING THE TWO CLAMP SECTIONSTOWARD ONE ANOTHER TO CLAMP SAID SURFACES OF THE SECTIONS AGAINST THESIDES OF THE PART ADJACENT BUT OUTWARDLY OF SAID END OF THE DIE TO GIVETHE PART AN INITIAL TRANSVERSE CURVATURE CORRESPONDING GENERALLY WITHSAID CURVATURE OF THE DIE.
 8. THE METHOD OF STRETCH FORMING A SHEETMETAL PART WHICH INCLUDES ENGAGING A SIDE OF THE PART AGAINST THE CURVEDFACE OF A DIE, GRIPPING THE ENDS OF THE PART WITH JAWS, GIVING THE PARTAN INITIAL TRANSVERSE CURVATURE CORRESPONDING GENERALLY WITH THETRANSVERSE CURVATURE OF THE END PORTIONS OF THE DIE BY GRIPPING ANDTRANSVERSELY BENDING THE PART IN REGIONS BETWEEN THE ENDS OF THE DIE ANDTHE JAWS, AND THEN MOVING THE JAWS TO SUBJECT THE PART TO ELONGATINGFORCES, WHILE SAID REGIONS OF THE PART REMAIN TRANSVERSELY BENT, TOSTRETCH FORM THE PART TO A CONTOUR DETERMINED BY THE SHAPE OF THE DIE.